The present invention relates to an apparatus for controlling the reference position of a recording paper in a multiple-time recorder, such as a polychrome recorder, in which recordings of different colors are superimposed on a single sheet of recording paper.
In a typical polychrome recorder, there is a thermal transcription recorder in which recording paper is displaced in a forward and also in a backward direction, sandwiched between a platen roller and a thermal head. (refer to FIG. 5) This displacement of paper is driven by a pulse motor and when a pulse is input, the paper is displaced by a unit displacement which is a very small quantity in a forward or in a backward direction. The drive direction is controlled by a different signal which is supplied to a motor control circuit.
Thus, in the transcription recorder, the recording paper is driven forward or backward in recording direction to change the relative position between the recording paper and the thermal head by a desired amount of displacement for recording on the recording paper.
The transcription recorder has an up-down counter for counting the number of pulses input to the motor control circuit. The up-down counter is reset at a reference position between the recording paper and the thermal head. The reference position is the base position at commencement of recording. A pulse for rotation in the forward direction is up-counted while a pulse for rotation in the backward direction is down-counted. The relative position of the recording paper and the thermal head can be indicated by the count value of the up-down counter, the count value of 0(zero) indicating the reference position.
There are provided ink ribbons of different colors, for example, an ink ribbon of yellow color, that of magenta color, that of cyan color, etc. When polychrome recording is performed, the paper is positioned at the reference position between the recording paper and the thermal head. An ink ribbon of, e.g., yellow color is overlaid on the paper and they are displaced in the forward direction to record a yellow color recording on the paper. When the yellow color recording is completed, the paper is returned to the reference position by displacing the paper in the backward direction. Then, the ink ribbon is changed to one having magenta color and the recording of magenta color is performed in the same way as mentioned above. As is apparent from the foregoing descriptions, it is indispensable to accurately accord the reference position of the recording in magenta color with that of the recording in yellow color. The position of the reference position is indicated by the count value of the up-down counter. However, the count value of the up-down counter may generate an error due to noise input. Moreover, due to the paper displacement of long distance in forward and backward directions fine slips between the platen roller and the paper are accumulated to generate an appreciable error. Therefore, another means to determine an accurate reference position is required in addition to the count value of the up-down counter.
In a prior art device, there is provided a preprinted alignment mark on a recording sheet to determine the reference position by reading an alignment mark by a sensor. In this method, there is a restriction that the paper on which the alignment mark is previously printed must be used. There may be an alternative way to print the alignment mark by the recorder itself. However, it is relatively difficult for the apparatus per se to print an accurate alignment mark. In either case, there is a problem that unnecessary alignment mark remains after the finish of the drawing.
In another prior art, the amount of the paper displacement in the forward or backward direction is converted into the rotational angle of a detection roller which rotates frictionally engaging with the paper. The rotational angle is converted into the number of pulses by an encoder and the number of pulses is counted by an up-down counter (i.e., up-counting for the forward paper displacement and down-counting for the backward paper displacement). From the count value of this up-down counter, the reference position is determined. However, there are such problems that slip between the paper and the detection roller generates an error and that the detection roller causes additional load to the paper displacement.
In another prior art, a paper edge is detected by a photosensor consisting of a pair of light-emitting element and light-receiving element to detect the reference position. In a case where a long recording paper such as a roll paper is used, however, there is a problem that the paper edge must be formed at an suitable portion of the paper.